It was hot. Of course, the entire summer had been hot, and there had been little rain. Rivers and ponds were drying up, water levels dropping. This was the case with the Kankakee River as we stepped up to the bank. The river was at least a couple feet lower than normal for this time of year—but we expected that, and it was one reason why we, a team of Shedd Aquarium fish biologists, were at a point on the river an hour south of Chicago, near the Illinois-Indiana state line. We were conducting a survey to see if the warm summer had impacted the fish community.
The water temperature was 90 degrees Fahrenheit instead of the typical mid-to-upper 70s as we stepped into the river with our nets. Because I’ve worked in the area for more than a decade, I know what fishes should be present, and there were plenty. We found 24 species and many individual animals across two places. We did not find any evidence of the “fish kills” rumored to be happening because of the warm weather.
But we found—or did not find—something else. Normally, the closely related northern pike and grass pickerel live alongside vegetation in the Kankakee River. The juveniles look very similar; hence, one has to look closely to identify them. In previous years, I found 50percent pike and 50 percent pickerel. This year we only found pickerel. Not a single pike! We found an above average number of gizzard shad, but no redhorse (elongate fish that swim along the bottom, eating insects that live in the mud and sand.)
Water temperature can explain all of these observations. Pickerel and gizzard shad better tolerate the stresses associated with warm water and could continue living in these habitats. Pike and redhorse prefer cool water and will die if the water temperature gets too high. In this case, they probably swam to a cooler part of the river, such as a deeper channel or hole. They will wait until the hot weather passes, then return to their previous habitats.
Another interesting observation was the high abundance of baby largemouth and smallmouth basses and baby bluegills. Normally these are found in shallow waters, like backwaters in the forests or side channels of the river. But this year, because of the low water levels, those habitats had dried up, forcing the fishes closer to the Kankakee’s main channel, where larger predators live. Right now, I think the baby basses and bluegills still have plenty of hiding places from predators, such as plants, rock piles and log jams. If the river level continues to drop, however, they will find it more and more difficult to avoid becoming someone else’s meal.
We also observed a nearby bayou. Usually it’s 200 yards long, filled with water and thick with aquatic plants. At the time of our visit, though, it had shrunk to a pool 40 feet across. Who knows how many fishes were originally in the bayou, but we found only two species that can tolerate low oxygen levels, bowfin and golden shiner. Bowfin can actually breathe air by using their swim bladder as a modified lung. We watched for several minutes as one after another rose to the surface for a gulp of air. But as tough as these fishes are, they cannot live in a stagnant pool forever. They need the rains to return and reconnect the pool to the river.
It is important to remember that rivers and their fishes are dynamic. They change daily, seasonally and from year to year. It’s not “bad” to find different species this year, or to see the river level fluctuate, or to watch temperatures rise and fall. It may feel strange to people, but it’s how nature works.
These fishes have survived in this river for thousands of years, through high water and low water, in scorching droughts and bitter cold snaps. As long as there are a variety of habitats, the fishes will find places of refuge during the hard times and return to their usual homes for the good times. Fishes are resilient to natural disturbances—as long as we do not severely pollute the river or destroy the variety of habitats.
Dr. Philip Willink, senior research biologist